Conventional linearly polarizing elements, such as linearly polarizing lenses for sunglasses and linearly polarizing filters, are typically formed from unilaterally stretched polymer sheets, which can optionally contain a dichroic material, such as a dichroic dye. Consequently, conventional linearly polarizing elements are static elements having a single, linearly polarizing state. Accordingly, when a conventional linearly polarizing element is exposed to either randomly polarized radiation or reflected radiation of the appropriate wavelength, some percentage of the radiation transmitted through the element is linearly polarized.
In addition, conventional linearly polarizing elements are typically tinted. Typically, conventional linearly polarizing elements contain a static or fixed coloring agent and have an absorption spectrum that does not vary in response to actinic radiation. The color of the conventional linearly polarizing element will depend upon the static coloring agent used to form the element, and most commonly, is a neutral color (for example, brown, blue, or gray). Thus, while conventional linearly polarizing elements are useful in reducing reflected light glare, because of their static tint, they are typically not well suited for use under low-light conditions. Further, because conventional linearly polarizing elements have only a single, tinted linearly polarizing state, they are limited in their ability to store or display information.
Conventional photochromic elements, such as photochromic lenses that are formed using conventional thermally reversible photochromic materials are generally capable of converting from a first state, for example a “clear state,” to a second state, for example a “colored state,” in response to actinic radiation, and reverting back to the first state in response to thermal energy. Thus, conventional photochromic elements are generally well suited for use in both low-light and bright conditions. Conventional photochromic elements, however, that do not include linearly polarizing filters are generally not capable of linearly polarizing radiation. The absorption ratio of conventional photochromic elements, in either state, is generally less than two. Therefore, conventional photochromic elements are not capable of reducing reflected light glare to the same extent as conventional linearly polarizing elements. In addition, conventional photochromic elements have a limited ability to store or display information.
Photochromic-dichroic compounds and materials have been developed that provide both photochromic properties and dichroic properties, if properly and at least sufficiently aligned. When in a colored or darkened state, such as when exposed to actinic light, photochromic-dichroic compounds, however, typically have a larger percent transmittance than non-polarizing or conventional photochromic compounds at equivalent concentrations and sample thickness. While not intending to be bound by any theory, and based on the evidence at hand, it is believed that the increased percent transmittance of photochromic-dichroic materials in the darkened or colored state is due to the percent transmittance being an average of the two orthogonal plane polarized components of the polarized radiation. A photochromic-dichroic material will more strongly absorb one of the two orthogonal plane polarized components of the incident random radiation, resulting in one of the planes of transmitted polarized light (passing through and out of the sample) having a greater percent transmittance than the other orthogonal plane polarized component. The average of the two orthogonal plane polarized components typically results in an average percent transmittance of greater magnitude. In general, as the linearly polarizing efficiency, which can be quantified in terms of absorption ratio, of photochromic-dichroic compounds increases, the percent transmittance associated therewith also increases.
It would be desirable to develop new polarizing photochromic articles that include photochromic-dichroic compounds, and which provide a combination of linear polarizing properties, and reduced percent transmittance when in a colored or darkened state, such as when exposed to actinic light. It would be further desirable that such newly developed polarizing photochromic articles have a combination of increased optical density and increased kinetics, such as increased fade rates, when exposed to a given amount of actinic radiation.